The invention is directed to a magnetic lens for controlling a charged particle beam, such as one directed by an instrument toward a very small spot on a sample for deriving characteristics of that sample and, in particular, to a winding coil arrangement that provides a faster adjustment of the magnetic flux and with improved resolution.
Various instruments are known that utilize a magnetic lens for controlling a charged particle beam directed toward a sample and which rely on interaction of charged particles emitted from that sample to derive characteristics thereof. Examples of such instruments are an electron microscope and a focused ion beam microscope. Other machines can also utilize a magnetic lens to direct a charged particle beam toward a sample, such as a lithography machine.
For facilitating the description of the present invention, it will be explained in connection with a scanning electron microscope (xe2x80x9cSEMxe2x80x9d). However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments and machines that require a focused beam of charged particles.
An SEM operates by generating a primary scanning electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and collected by a detector which is arranged near the surface of the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is used as an input to various well known electronic components to output signals used for displaying an image of the surface on a video screen.
A typical arrangement of the main components of an SEM is schematically shown in FIG. 1. Electron source 2 generates an electron beam 3 which is directed through aligned openings at opposite ends of tube 4 toward sample 5. Detector 6 collects electrons emitted from sample 5. Beam 3 passes through opening 8 in detector 6. Beam 3 is controlled by stigmation coils 7, alignment coils 9, scan coils 11a and 11b, and lens 13. The function of these components is well known. Briefly, stigmation coils 7 are used to correct the shape of the beam. Alignment coils 9 are used to align the beam through the tube 4. Scan coils 11a and 11b deflect electron beam 3 in two directions, respectively, such as along an x-direction and a y-direction in a plane perpendicular to the beam direction. SEM""s can contain more than one of any of these components, as well as other components that are not described herein. Also, the positions of the various components need not be as shown in FIG. 1 which is presented for illustrative purposes rather than accuracy.
Lens 13 is an electromagnetic lens which is provided for focusing of the beam 3 to a very small spot to enable high resolution imaging. Such a lens is commonly called an objective lens. In the illustrative representation of objective lens 13 in FIG. 1, it includes a toroidal, channel-shaped magnetic polepiece 14 with a lens inner pole 15 and a lens outer pole 17, and a winding 19 inside the channel.
FIG. 2 shows a cross sectional view of a specific type of objective lens 30. The detailed description of the present invention which follows below will be with respect to this type of lens 30, although it will be understood that in no way does such description limit the invention to use on only that lens.
Briefly, lens 30 has a toroidal, channel-shaped magnetic pole 40. Pole 40 has an inner yoke 42, an outer yoke 44, and a winding 46 inside the channel. The inner and outer yokes 42 and 44 are provided with poles 50 and 60 that project toward sample 5 and serve to create a magnetic flux pattern which focuses beam 3 to a very small spot as it impacts the sample.
Although objective lens 30 is effective for focusing the electron beam 3, when an adjustment of the magnetic flux is required to conduct calibration and various other lens operations, its time constant is too long for certain purposes. Also, when a relatively small adjustment is required, its adjustment resolution may not be sufficient.
One object of the invention is to provide a winding coil arrangement for a magnetic lens that enables a faster adjustment of the magnetic flux.
Another object of the invention is to provide a winding coil arrangement that provides adjustment of the magnetic flux with improved resolution.
These and other objects are attained in accordance with one aspect of the present invention directed to a winding coil for a magnetic lens which controls a charged particle beam. The winding coil includes a bobbin arrangement, a primary winding wound on the bobbin arrangement and having a selected number of ampere-turns, and a secondary winding wound on the bobbin arrangement and having a selected number of ampere-turns. The number of ampere-turns of the secondary winding is substantially smaller than the number of ampere-turns of the primary winding.